Over the recent years, Ethernet (registered trademark) has been utilized not only as LAN (Local Area Network) but also as a provider network as an infrastructure technology which supports a packet network. Provider Backbone Bridges (PBB) standardized in IEEE 802.1ah is a technology used for a provider to provide a service for transmitting a Media Access Control (MAC) frame to a customer via a provider network which employs Ethernet (registered trademark). According to the PBB, a customer MAC frame (which is referred to as a “service frame”) that is transmitted and received between customer points (lodgments) is encapsulated with another MAC frame (MAC-in-MAC encapsulation) in the provider network linking up the customer points to each other and is forwarded within the provider network. The MAC frame with which the service frame is encapsulated is called a “backbone MAC frame”.
FIG. 11 is an explanatory view of a conventional example of a service providing network for providing a MAC frame transmission servicer by use of the PBB. The example depicted in FIG. 11 is that the service providing network includes a provider network including a plurality of provider edge (PE) apparatuses. Customer edge apparatuses (CE) serving as the customer points are coupled to the respective provider edge apparatuses (PE). Thus, each of the CEs can receive a MAC frame forwarding service via the provider network in a case of transmitting the MAC frame to another CE.
The service frame, which is transmitted from the customer edge apparatus (e.g., CE#12 in FIG. 11) on a transmission side and addressed to another customer edge apparatus (e.g., CE#13), contains a payload (user data), a MAC source address (SA) and a MAC destination address (DA). The service frame is attached with a service instance identifier (ISID or I-SID) associated with the service and a backbone VLAN (Virtual Local Area Network) identifier (BVID or B-VID) in the provider edge apparatus (which is called an ingress edge node, e.g., PE#12 in FIG. 11) located at an ingress of the provider network.
Further, the ingress edge node (PE#12) attaches, to the service frame, a backbone MAC source address (which is abbreviated to BSA or “B-SA”) associated with the ingress edge node as well as attaching a proper backbone MAC destination address (which is abbreviated to BDA or “B-DA”) by referring to the MAC destination address (DA) of the service frame, and sends the service frame to within the provider network. This type of backbone MAC frame is transmitted to within the provider network.
Then, the backbone MAC frame reaches, based on the BDA, an egress edge node (which is a provider edge apparatus on an egress side, e.g., PE#13 in FIG. 11) of the provider network. The egress edge node restores the backbone MAC frame back into the original service frame (decapsulation) by removing the BDA, the BSA, the BVID and the ISID therefrom, and sends the decapsulated service frame to the reception-sided customer edge apparatus (corresponding to CE#13 in FIG. 11) associated therewith. Furthermore, the egress edge node learns a route of the backbone MAC frame from the SA and the BSA contained in the backbone MAC frame.
By the way, it is much of importance to improve availability of the service in terms of providing the service or transmitting the MAC frame. For example, it is considered to adopt a configuration called “multihoming” by way of one means for improving the availability. In a case where the multihoming is applied, a plurality of access lines is prepared between one single customer edge apparatus being a customer point and the provider edge apparatuses, and further the customer edge apparatus is coupled to the plurality of provider edge apparatuses via the plurality of access lines (refer to, e.g., Patent document 1 and Patent document 2).
FIG. 12 is an explanatory view of a conventional example of realizing the multihoming in the service providing network by use of the PBB and of a problem arising at this time. The customer edge apparatus CE#13 is coupled to the provider network (PBB network) via the provider edge apparatuses PE#13 and PE#14 by employing two access lines AL13 or AL14. Adoption of such a configuration enables continuation of providing the service against failures in the access lines AL13 and AL14 and a failure in the provider edge apparatus PE#13 or PE#14 and also enables the availability to be improved.
In the case of adopting the network architecture illustrated in FIG. 12, such a network topology is configured as to take a loop topology consisting of the customer edge apparatus CE#13, the provider edge apparatuses PE#13, the PBB network serving as the provider network and the provider edge apparatus PE#14.
When this type of loop topology is configured, as depicted in FIG. 12, such a problem of loop of service frames arises that the MAC frame (service frame) transmitted from the customer edge apparatus CE#13 is returned (loopback) again to the customer edge apparatus CE#13 via the provider network (PBB network).
Further, there is a case in which a provider edge apparatus PE of the transmission destination, which is associated with a designated destination address (MAC DA) of the service frame sent from the customer edge apparatus CE#11 and received by the provider edge apparatus PE#11, is unknown. In this case, the provider edge apparatus PE#11 performs flooding of the service frames to all other provider edge apparatuses PE belonging to the same service. At this time, if the multihoming configuration depicted in FIG. 12 is adopted, such a problem occurs that the multiple service frames redundantly reach the customer edge apparatus CE#13, namely, the same service frames redundantly reach the customer edge apparatus CE#13.
The network is built up so as not to logically configure the loop topology of the network in order to avoid the occurrence of the problem described above. For instance, with respect to a case where the loop topology exists in the physical network topology, Spanning Tree Protocol (STP) specified in IEEE Standard 802.1D or Rapid STP (RSTP) defined as an improved version of the STP or Multiple STP (MSTP) is proposed as a method of configuring a loopless logical network topology.
FIG. 13 is an explanatory view of the conventional example which uses the STP. FIG. 13 illustrates an example of implementing such setting as to block a port, on the side of the customer edge apparatus CE#13, of the provider edge apparatus PE#13. With this setting, the loop topology is eliminated from on the logical network topology. Accordingly, the looping problem and the redundant arrival problem of the service frames are avoided.
[Patent document 1] Japanese Patent Application Laid-Open Publication No. 2007-43678
[Patent document 2] Japanese Patent Application Laid-Open Publication No. 2008-312191